Force sensing devices such as, for example, piezo-resistive force sensors have become more popular in electro-mechanical systems because they provide precise and reliable force sensing performance in a compact package. Furthermore, the compact package of the piezo-resistive force sensor facilitates easy integration into the electro-mechanical systems. The piezo-resistive force sensor operates on the principle that the silicon implanted piezo-resistors vary their resistance when deformed by a force. Typically, these piezo-resistors are fabricated in a silicon die/chip, where the silicon die/chip is configured to flex or compress by such forces.
Vehicles, as transport means, have become an important part of society. Typically, driving a vehicle requires the constant use of brakes to slow down or stop the vehicle. The inefficiency of a braking system can result in traffic accidents and other undesirable consequences. Therefore, knowledge and control of the braking force is important to improve the efficiency of the braking system. The piezo-resistive force sensors are integrated in vehicle braking systems to measure and facilitate control of the braking force, thus improving the overall braking performance.
FIG. 1 illustrates a piezo-resistive force sensor 1 for use in, for example, brake applications. The force sensor 1 includes a plunger 2, a flexible membrane 4 a silicon die 10, and a support plate 7. The plunger 2 is configured to receive a force from a load, wherein the plunger 2 transmits the force to the membrane 4. In particular, the lower surface of the plunger 2 has a ring-shaped bump 3 that complementarily fits into a ring-shaped groove 5 at the upper surface of the membrane 4. The membrane 4 is supported by the fixed support plate 7, wherein the lower surface of the membrane 4 has another ring-shaped groove 6 configured to allow a ring-shaped bump 8 on the support plate 7 to fit into. Furthermore, the lower surface of the membrane 4 has a centrally disposed convex step 9 that secures the silicon die 10 between the membrane 4 and the support plate 7.
The configuration of the force sensor 1 works under the principles of a lever mechanism, wherein the bump 8 on the support plate 7 acts as a fulcrum element for the membrane 4, and wherein the membrane 4 acts as a lever element. When the plunger 2 receives a force from the load, the force is transmitted to the membrane 4, causing the membrane 4 to flex. The flexing of the membrane 4 bends and/or compresses the silicon die 10, thus resulting in the silicon die 10 making a measurement of the force from the load. During this operation, a large proportion of the force applied on the membrane 4 is transmitted from the groove 5 to the groove 6. The force applied on both grooves (5, 6) of the membrane 4 creates a high level of stress on the membrane 4. Thus, the membrane 4 is easily subjected to fracture and other stress-induced deformation which reduce the durability and reliability of the structure. Furthermore, the accuracy of the force measurement also decreases.
There is therefore a need for durable and reliable piezo-resistive force sensors to provide precise measurements of force.